It is not too much to say that the modern society is being maintained by a large amount of electrical energy. Lots of people breathe in pleasant air in buildings by the aid of a conditioning system, talk to one another in different places using mobile phones, and moves to desired places by subways. In the modern society, electricity is a main energy source being widely used in every field of life although its significance is not recognized like water and air.
A majority of electrical energy is obtained by power generation systems using fossil fuels. However, recently, due to the exhaustion of fossil fuels and environmental pollution, attention is being paid to power generation systems using new renewable energy that produce electrical energy without using fossil energy.
New renewable energy refers to energy existing in nature, such as sunlight, solar heat, wind, tides, waves, geothermal heat, and the like. Power generation systems using new renewable energy are gaining popularity in that they do not involve resource exhaustion and environmental pollution issues. Among the power generation systems using new renewable energy, a wind power generation system uses wind energy.
A wind power generation system is a system that generates power using a wind power generator connected to and operated by a windmill that rotates by the wind and runs the wind power generator by its rotating power after being installed in an area in which the wind blows at more than a predetermined strength. To generate power stably using the wind power generation system, the wind power generator is installed in an area in which the wind blows continuously over the year. Also, for uniform quality of power generated by wind, the wind power generator is preferably installed in an area in which the wind blows at a uniform strength so much that a change in wind strength is not too great.
However, because wind blow is a natural phenomenon, it is very difficult to find an area perfectly satisfying all the desired conditions. Particularly, as wind strength is not uniform, a rotation speed of a windmill constantly fluctuates. In response to this phenomenon, techniques for changing a blade angle of a windmill based on wind strength are used. When wind strength is too low, a blade angle is adjusted to increase an area of a windmill blade facing the wind. In contrast, when wind strength is too high, a blade angle is adjusted to reduce an area of a windmill blade facing the wind. However, the techniques for changing the blade angle are inadequate for a small change in wind strength. Accordingly, to maintain the quality of power, a secondary battery is used together.
FIG. 1 is a block diagram schematically illustrating a configuration of a wind power generation system 100 according to a related art.
Referring to FIG. 1, the wind power generation system 100 includes a power generation unit 110, a power storage unit 120, and a power control unit 130. Also, the power generation unit 110, and the power storage unit 120, and the power control unit 130 are connected to one another through a power line 150. The power line 150 provides a channel for power transfer that may also be used as a path for conveying a control signal to run the wind power generation system 100. In FIG. 1, a schematic illustration is provided to depict the power generation unit 110, and the power storage unit 120, and the power control unit 130 electrically connected to one another.
The wind power generation system 100 is a system that generates power using wind energy, one form of new renewable energy, and supplies the generated power to a power grid 140. The power grid 140 may be a commercial power grid or a small-scale local power grid. Also, according to circumstances, the power grid 140 may be a power storage device that stores power within a smart grid, a load that consumes generated power in an instant, or a power conversion device.
The power generation unit 110 serves to convert wind energy into electrical energy. For this, the power generation unit 110 includes a windmill that rotates by the wind, and a power generator operable to generate power by the rotating power of the windmill. Since principles of wind power generation are widely known to a person having ordinary skill in the art to which the present disclosure pertains, hereinafter referred to as a skilled person, a detailed description of specific construction is omitted herein.
The power storage unit 120 stores a certain amount of power generated by the power generation unit 110 by a control signal of the power control unit 130, or discharges a certain amount of stored power by a control signal of the power control unit 130. For this, the power storage unit 120 includes a secondary battery 121 capable of storing and discharging power.
In the supply of power generated by the wind power generation system 100 to the power grid 140, the power control unit 130 controls an amount of power supplied to the power grid 140 for quality stabilization of power. The term “quality stabilization of power” as used herein represents maintenance of a power factor required by the wind power generation system 100, for example, voltage and current of power supplied from the wind power generation system 100, and in the case of alternating current output, a frequency and a phase.
As described in the foregoing, because wind blow is a natural phenomenon, wind strength may be ununiform. Thus, a rotation speed of the windmill constantly fluctuates. The rotation speed of the windmill directly affects a rotation speed of the generator, and the rotation speed of the generator is directly related to the quality of output power. Basically, the quality of output power is stabilized by changing a blade angle of the windmill based on the wind strength, but in the case of a small change in wind strength, the quality of output power is stabilized through the power storage unit 120.
FIGS. 2 through 4 are conceptual diagrams illustrating quality stabilization of power.
First, referring to FIG. 2, an arrow indicated below the power line 150 denotes a concept of the supply of power generated by the power generation unit 110 to the power grid 140. Also, a figure indicated within the arrow denotes an amount of power generated by the power generation unit 110 being supplied to the power grid 140. In this instance, 100% represents that an amount of power generated by the power generation unit 110 completely satisfies an amount of power required by the power grid 140 or an amount of power scheduled to be generated by the power generation unit 110. Accordingly, in case the power generation unit 110 generates power in a full amount of power required or scheduled, there is no need for the power storage unit 120 to perform charging or discharging.
Next, referring to FIG. 3, a case in which an amount of power generated by the power generation unit 110 is larger than an amount of power required or scheduled by the power grid 140 is illustrated. When seeing FIG. 3 in comparison to FIG. 2, the power generation unit 110 generates power by 110% of the amount of power required or scheduled by the power grid 140. In this case, the power control unit 130 outputs a charge control signal to store an amount of power in excess (10%) in the power storage unit 120. Thereby, an amount of power supplied to the power grid 140 meets 100%.
Next, referring to FIG. 4, a case in which an amount of power generated by the power generation unit 110 is less than an amount of power required or scheduled by the power grid 140 is illustrated. When seeing FIG. 4 in comparison to FIG. 2, the power generation unit 110 generates power by 90% of the amount of power required or scheduled by the power grid 140. In this case, the power control unit 130 outputs a discharge control signal to discharge an amount of power in short supply (10%) from the power storage unit 120. Thereby, an amount of power supplied to the power grid 140 meets 100%.
As described in the foregoing, when an amount of power generated by the power generation unit 110 is larger than an amount of power required or scheduled, the power control unit 130 stores, in the power storage unit 120, an amount of power in excess among the amount of power generated by the power generation unit 110. In contrast, when an amount of power generated by the power generation unit 110 is less than an amount of power required or scheduled, the power control unit 130 discharges, from the power storage unit 120, an amount of power in short supply among the amount of power generated by the power generation unit 110. In this way, the power control unit 130 stabilizes the quality of power supplied to the power grid 140.
Although the wind power generation system 100 was described as an example among power generation systems that generate power using new renewable energy, other types of power generation systems also use the secondary battery 121 to improve the quality of power.
Meanwhile, the secondary battery 121 has an upper limit at which the secondary battery 121 can be physically charged and a lower limit at which the secondary battery 121 can be physically discharged. However, in an actual usage environment, the secondary battery 121 is not charged and discharged to the physical upper and lower limits. That is, a lower limit of a use area is set to be higher than a physical discharge limit point, and an upper limit of a use area is set to be lower than a physical charge limit point. Accordingly, when a state of the secondary battery 121 reaches the set lower limit of the use area during discharging of the secondary battery 121, the secondary battery 121 is said to be fully discharged. Also, when a state of the secondary battery 121 reaches the set upper limit of the use area during charging of the secondary battery 121, the secondary battery 121 is said to be fully charged. For example, the use area of the general lithium secondary battery 121 is set based on voltage, and has a voltage interval from 3.7V to 4.2V.
However, it is preferred that the secondary battery 121 included in the power storage unit 120 maintains a constant charge amount within the range between full charge and full discharge. This is because it is difficult to predict when an amount of power generated by the power generation unit 110 will fall short or go over. Accordingly, the power control unit 130 controls a charge amount of the secondary battery 121 to maintain at a constant amount to allow for both charging and discharging by the power storage unit 120. That is, there is a need to research into an apparatus and method for controlling charge and discharge to maintain a charge amount of the secondary battery 121 to prevent the secondary battery 121 from being fully discharged.